A Syria-bound Russian military plane crashed into the Black Sea Sunday, with no sign of survivors among the 92 on board, including Red Army Choir members travelling to celebrate New Year with troops.

The Tu-154 plane went down shortly after taking off from the southern city of Adler where it had been refueling, defense ministry spokesman Igor Konashenkov said.

It disappeared from radar just two minutes after it took off at 5:25 am (0225 GMT).

The ministry told agencies there was no sign of any survivors at the crash site and that 10 bodies had been recovered off the coast of the resort city of Sochi, as authorities pledged to dispatch an additional 100 divers to aid in the search.

"Fragments of the Tu-154 plane of the Russian defense ministry were found 1.5 kilometers (0.9 miles) from the Black Sea coast of the city of Sochi at a depth of 50 to 70 meters (165 to 230 feet)," the ministry said.

President Vladimir Putin told state television that Russia will observe a day of national mourning on Monday.

The plane had been on a routine flight to Russia's Hmeimim air base in western Syria, which has been used to launch air strikes in Moscow's military campaign supporting its ally President Bashar al-Assad in the country's devastating civil war.

Among the plane's 84 passengers were Russian servicemen as well as 64 members of the Alexandrov Ensemble, the army's official musical group also known as the Red Army Choir, and its conductor Valery Khalilov.

They were headed to Syria to participate in New Year celebrations at the air base.

There were also nine journalists and eight crew onboard.

Mourners laid flowers and candles throughout the day in front of the Moscow concert hall where the Red Army Choir usually performs in the Russian capital.

Transport Minister Maxim Sokolov, in charge of a government probe into the crash, said on state television that investigators were looking into a "whole spectrum" of theories on the cause of the crash.

When asked if a terror attack could have been behind the crash, Sokolov said: "It is premature to speak of this."

He added that the aircraft's black boxes had yet to be located.

The defense ministry said the searches would go on round-the-clock with the help of ships equipped with searchlights.

There were also nine journalists and eight crew onboard.

A list of passengers published by the defense ministry also included Elizaveta Glinka, a doctor and charity worker who serves on the Kremlin human rights council.

Probing cause of crash

The US joined German Chancellor Angela Merkel and Turkish Prime Minister Binali Yildirim in expressing condolences over the crash.

Kremlin spokesman Dmitry Peskov said that Putin was being kept updated on the search operation and was in constant contact with Defense Minister Sergei Shoigu.

Russian army spokesman Igor Konashenkov said that more than 3,000 people, 32 vessels, about 100 divers and five helicopters were taking part in the search operation.

Pictures from the scene showed rescue workers carrying bodies on stretchers on a pier in Sochi.

The transport ministry said the bodies recovered from the crash site would be sent to Moscow for identification.

According to Konashenkov, the aircraft had been in service since 1983 and had flown some 7,000 hours since. The plane last underwent repairs in December 2014 and was serviced in September, he said.

Russia's Investigative Committee said a criminal probe had been launched to determine whether violations of air transport safety regulations had led to the crash.

Indefinite deployment

Investigators are currently questioning the technical personnel responsible for preparing the plane for take-off, the committee said.

Tu-154 aircraft have been involved in a number of accidents in the past. But Sergei Bainetov, the air force head of flight safety, said the plane was "in good condition technically".

In April 2010 many high-ranking Polish officials, including then president Lech Kaczynski, were killed when a Tu-154 airliner went down in thick fog while approaching Smolensk airport in western Russia.

Moscow has been conducting a bombing campaign in Syria in support of Assad since September 2015.